Component casting is used in order to produce a wide range of components and members. Essentially, the component is cast in a mould from a molten liquid and then allowed to cool in order to leave a solidified component. Some components such as turbine blades for jet engines require structural abilities such as high temperature creep resistance. This is achieved with turbine blades through forming a single crystal. At high temperatures, typically above half the absolute melting temperatures of the metal, grain boundaries within the metal become weaker than the grain bodies they surround such that the absence of grain boundaries in a single crystal provides resistance to creep.
Techniques for producing single crystal components are well known. Essentially the component is cast in a mould and then gradually withdrawn from a furnace in an appropriate manner such that propagation of a single crystal is achieved. Typically, a so called “pig-tail” selector is used in order to initiate a single grain or crystal growth. The most important consideration with respect to continued propagation of a single crystal within the component is to ensure so called directional solidification. This is achieved by gradual withdrawal, usually downwardly of the component from the furnace such that the temperature gradient is effectively controlled.
A preferred method of component casting is that known as the lost wax process. This is a traditional technique in which a component is initially formed as a wax structure and then a ceramic coat is placed upon that wax structure and allowed to harden. The wax is then removed, typically by heating, in order to leave the ceramic as a mould for the component. As indicated above, the component is cast from a molten liquid and then allowed to cool and solidify.
The bottom filling of seeded single crystal investment castings is common practice on large chill plate castings where there is sufficient room to locate filters and runners which feed molten metal from an upper pour cup or basin to the seed crystal. A typical casting arrangement 10 used on a large chill mould is shown in FIG. 1. The arrangement 10 includes a chill base 12 which supports an upstanding downpole 14 (or sprue). Mounted on top of the downpole 14 is a downwardly pointing conical pour cup 16 (or basin). A runner 18 in the form of a pipe extends downwardly from the pour cup 16 at a point towards the apex thereof, to feed molten metal to a mould. A filter 20 is provided in line with the runner 18, towards the top end of the runner 18.
The lower end of the runner 18 connects to the lower end of a mould such that molten metal extends over a seed crystal 22 and extends up into a spiral grain selector 24, which leads into a mould to produce a turbine blade 26 as shown. The mould in which the turbine blade 26 will be formed is not shown in FIG. 1. It is to be realised that a number of moulds and corresponding runners 18 will be provided spaced around the downpole 14, and typically for instance four blades may be made in a single arrangement 10.
Such an arrangement can generally only be applied to large chill furnaces in which there is sufficient room to provide the respective runners 18 and filters 20. With such a bottom filling arrangement the wax assembly is relatively difficult and the lead times are generally longer. The seed crystals 22 have to be fitted after shelling thereby adding a further manufacturing operation, increasing lead times and giving an opportunity for further non conformance due to poor fitting of the seed, contamination by ceramic adhesive, or the seeds actually falling out during handling of the moulds.